The present invention relates to an apparatus for measurement of critical current in superconductive tapes and to a method of identifying areas of superconductive tapes with lower critical current properties.
Since their initial development, coated conductor research has focused on fabricating increasing lengths of the material, while increasing the overall critical current carrying capacity. Early samples were typically fabricated by stationary processes where the substrates were held in one position in the various deposition chambers and the various buffer layers, YBCO, and finally silver were deposited. This technique could produce samples of about 10 centimeters (cm) in length and was therefore limited. Consequently, characterization techniques focused on making one or two critical current measurements on a sample. Such coated conductor fabrication requires cryogenic testing of current carrying capacity as part of the production cycle. This measure of current carrying capacity is called xe2x80x9ccritical currentxe2x80x9d and is abbreviated as Ic, measured in Amperes.
These measurements first entailed soldering a current contact on either end of the sample using In97/3Ag (indium/silver) solder, and soldering 2 or more voltage taps spaced evenly at 1 cm intervals on the sample. The experimenter would then make an end to end measurement of the critical current by ramping the current through the sample while monitoring the voltage across the two most extreme voltage taps. The critical current was then defined as the current needed to develop a voltage of 1 xcexcV per centimeter of sample between the two voltage contacts. Likewise, the critical current homogeneity was measured by ramping current through the entire sample while monitoring the voltage developed across consecutive pairs of voltage contacts. In 1995 this measurement technique yielded an Ic result of about 200 amperes measured across 1 cm.
With the development of continuous deposition techniques, lengths up to 1.1 meters of coated conductor are now produced. The production of these longer lengths required the development of new measurement techniques. Where once, characterization was done by soldering voltage taps on the samples, problems due to the effects of soldering to the silver coating necessitated the development of solderless contacts. Concurrently, the need to characterize the longer lengths drove the measurement process to consist of not just a few Ic measurements on a scale of 1 cm, but an end to end measurement over as long a length as possible, and then a series of over 100 Ic measurements made on a 1-cm scale to determine the critical current homogeneity of the sample. This was accomplished by holding the sample down on a G10 substrate with 100 voltage contacts consisting of 1-cm wide pieces of copper-beryllium finger stock. The finger stock was wired by means of 4 lengths of 25 conductor ribbon cable to a patch panel on which were mounted 50 banana bulkhead connectors. The entire sample and G10 assembly was then immersed in liquid nitrogen. The experimentalist would then do an end to end measurement and then proceed measure the critical current homogeneity of the sample by working their way through the consecutive pairs of connectors on the panel. The first 1-meter long samples measured in this manner had end to end critical currents of between 0.1 and 4 amperes, yet certain sections had critical currents as high as 45 amperes. FIG. 1 shows the critical current homogeneity of two 1-meter long samples.
The inferior sections had to be capable of carrying large currents during the measurement of the superior sections without burning out. Thus increasingly thick layers of silver were deposited to perform as current contacts as well as to carry current over inferior sections during the measurement of the high critical current sections. This occasionally resulted in the destruction of the sample during the measurement. Yet measurements continued in this manner until recently when process improvements resulted in tape being fabricated which carried close to 110 amperes of super current. The present record of an Ic of 125 amperes measured over 100 cm and an Ic of 175 amperes on the same sample measured over 67 ushered in a series of samples for which determination of the homogeneity using the aforementioned method was impossible. These recent increases in critical current capacity have spurred development of a new technique for the characterization of sample homogeneity.
Despite the recent progress in production of superconductive tapes, improvements have been desirable in the measurement of critical current properties.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention provides an apparatus for measurement of critical current in a superconductive tape including a means for applying a localized magnetic field to a portion of a superconductive tape, a means for measuring critical current of the portion of the superconductive tape subjected to the localized magnetic field, and, a means for positionally locating to specific portions of the superconductive tape both said means for applying a localized magnetic field and said means for measuring critical current.
The present invention further provides a process for measuring critical current in a superconductive tape in a manner capable of determining critical current variations between varying regions of the superconductive tape including applying a localized magnetic field to a first portion of a superconductive tape, measuring critical current of the first portion of the superconductive tape subjected to the localized magnetic field, repositioning said localized magnetic field to a second portion of a superconductive tape, measuring critical current of the second portion of the superconductive tape subjected to the localized magnetic field so as to provide a critical current mapping of multiple portions of the superconductive tape.